Conventional propelled machines may generally include an internal combustion engine that is mechanically coupled through a transmission assembly and drive train to the driven wheels or sprockets of the machine. In contrast, propelled machines having electric drive systems include an internal combustion engine that is mechanically coupled to drive a generator that creates electrical power. The power from the generator is then consumed by a motor that is mechanically coupled to drive the wheels or sprockets of the propelled machine. Accordingly, the generator and motor of the electric drive systems can replace the mechanical transmission and drive train of conventional internal combustion engine driven machines. This may result in a propelled machine with superior drive train efficiency and improved propulsion performance, which correlates to a machine having greater fuel economy and reduced emissions.
In one type of conventional mechanical transmission system, a gear selector allows the operator to select between various directions of travel (e.g., forward, neutral, reverse, etc.) and various gear ratios (e.g., first, second, third, fourth gear, etc.) of the mechanical transmission. Machine speed may be controlled by selecting the gear ratio of the transmission. Each gear ratio may correlate, for example, to a predetermined speed range. On the other hand, electric drives are not limited to a finite number of gear ratios and therefore do not provide a similar feature of selecting gear ratios corresponding to predetermined speed ranges.
Furthermore, in one type of a conventional mechanical transmission system, an impeller clutch is connected between the engine and the transmission of the machine, and is actuated through an impeller clutch pedal. When the impeller clutch pedal is depressed, the impeller clutch leaves an engaged state where the full amount of power is transmitted from the engine to the transmission to a disengaged state where the engine power is variably passed to the transmission based on the amount of pedal depression. Upon further depression of the impeller clutch pedal, the impeller clutch decouples the engine from the transmission, and the brakes are engaged to slow the speed of the machine. As a result, the machine's ground speed is reduced for more precise control while the engine speed is maintained at a relatively high rate to enable rapid response of auxiliary equipment operated by the engine. However, with the impeller clutch, power may be dissipated as heat, thereby decreasing drive train efficiency, and it may be more difficult to control the transfer of power to the ground.
U.S. Pat. No. 6,492,785 (the '785 patent) to Kasten et al. describes using an electric drive system instead of a mechanical transmission system with a clutch pedal for reducing torque. The electric drive system of the '785 patent includes a pedal connected to a transducer that forms a clutch command signal associated with the position of the pedal. As the pedal is depressed, the clutch command signal reduces the current supplied to the motor until the torque reaches zero at a nearly fully depressed position of the pedal. Although the system of the '785 patent may disclose using a clutch pedal to limit motor torque, this may also be insufficient to limit the undesired power of the machine as described above.
The disclosed system is directed to overcoming one or more of the problems set forth above.